For many years electricity has been produced using boilers or furnaces which generate steam that drives a turbine. Many of the furnaces used to produce electricity have walls formed by laterally adjacent water filled tubes welded together. The tubes may abut one another and be welded together along the line of abutment. Alternatively, the tubes often are connected together by webs, one web between each pair of adjacent tubes. These furnace walls are sometimes called water walls. The water in the tubes is heated by conductive heat transfer. The heated water may reach temperatures of from 700° F. to 1100° F. The water is under pressure and is used to drive a turbine to produce electricity. Depending upon the type of furnace, the water in the tubes may be at pressures of from 2000 pounds per square inch to as high as 5200 pounds per square inch. Most furnaces being used today are operated twenty four hours a day, seven days a week for six to nine months and sometimes 18 months, 24 months or even longer. Then the furnace is shut down for cleaning and routine maintenance. Such maintenance is almost always scheduled in advance to assure that other furnaces are available to supply any needs that may occur during shutdown. Consequently, there is no access to the interior of most furnaces during most of the year.
It is desirable to measure the heat flux into these walls as ash accumulates on them. If the ash remains solid in contact with the wall, it may accumulate, thus effectively insulating the water wall from its heat source and defeating the purpose or efficiency of the boiler. If it melts on the wall, the liquid ash may cause corrosion. When the ash is liquid, it is generally referred to as fused ash, vitrified ash, or most commonly as slag.
Furnace wall tubes are usually made from iron containing metal alloys which often contain 1-5% chromium. During operation of the furnace a protective iron oxide film forms on the fire side surface of the tubes. Ash particles and slag also accumulate on top of the iron oxide film. That slag can be a solution or mixture of iron and silicon oxides, which is commonly identified as FexOySiO2. Other chemicals, particularly calcium and aluminum may also be present in the slag. Depending upon oxidizing or reducing conditions and the relative amounts of calcium, iron and silicon present in the slag, and also the presence of potassium and/or phosphate aluminates, the slag will be either liquid or solid at operating temperatures within the furnace.
Until recent years furnace wall tubes corroded slowly because of the protective oxide layer and had a service life of many years, often greater than 20 years throughout the furnace. However, the introduction of low NOx burners has increased the rate of corrosion of these tubes, which can reduce their life expectancy. The result is that not only do tubes have to be replaced, but the corrosion problem has also resulted in the need to improve coal quality, or ash fusion characteristics, sometimes doubling the cost of coal. Consequently, there is a need for a method that will reduce corrosion of furnace wall tubes in boilers. Such a method must first identify when and where corrosion is occurring. Then adjustments can be made to eliminate or reduce the corrosion causing conditions.
Because the water inside tubes is at a high pressure, the tubes could fail if their walls become too thin as a result of corrosion. For this reason, the industry has periodically measured the thickness of the walls of its tubes using sonic measuring techniques and other methods. When these measurements indicate that the walls are becoming too thin, the tubes are replaced. While the industry has been able to determine corrosion rates from periodic measurements of wall thickness, corrosion rates determined in this way are of little use in efforts to control corrosion. That is so because the measurement intervals are such that significant corrosion has occurred between measurements and further this significant corrosion does not shed light on the variation of conditions, mechanisms, coal properties or operating adjustments which cause the significant corrosion.
The corrosion of furnace wall tubes involves several mechanisms. First, removal of the protective oxide film allows further oxidation. Second, if the oxide film is not present or is in an oxidizing-to-reducing transition the iron surface is attacked and pitted by condensed phase chlorides, which may be present. A third mechanism occurs when wet slag runs across the surface of the oxide film. As that happens, iron from the tube goes into the slag solution, particularly when it is in a reducing or oxygen starved condition that can be caused by low NOx firing. Low fusion calcium-iron-silicate eutectics, alkali iron trisulfates, and sodium vanadates will have formed in the liquid slag. Reduced sulfur in the form of S, H2S, FeS or FeS2 can react with the oxygen of the tube scale depriving the tube metal of its protective layer. Vanadium has different valence states that allow liquid sodium vanadate to react with oxygen from the flue gas. That reaction raises the vanadium oxidation state. Oxygen is deposited on the iron forming rust (FeO, Fe2O3, Fe3O4) reducing the vanadium oxidation state. If one could detect when corrosion is occurring, then steps could be taken to stop such corrosion. Yet, prior to the present invention the art has not been able to identify when and where corrosion is occurring while the furnace is in operation. Therefore, it has not been possible to make adjustments to the furnace operation to reduce or stop that corrosion.
Flue gas within the stack as well as flue gas within the furnace is not a homogeneous mixture. The identity and amount of combustion products that are present at any particular time and place in the furnace will typically be different from the identity and amount of combustion products that are present at other places within the furnace or at the same place but at a different time during furnace operation. Consequently, conditions that favor corrosion may be present in some furnace regions but not in others. Hence, corrosion may be occurring on one portion of a furnace wall but not on other portions of the same wall. Frequently, these differences are attributable to the fact that there are several burners in the furnace and one or more burners is operating differently from other burners. If that be true, it should be possible to reduce or eliminate the corrosion by adjusting the operation of one or more burners.
The art has long recognized that slag build-up on furnace walls reduces heat transfer from the combustion chamber to water in the tubes. Therefore, the art has monitored slag build-up and provided soot blowers to remove such build-up during operation of the furnace. Soot blower systems typically use thermocouples to measure furnace wall temperature. Jonakin et al. in U.S. Pat. No. 3,276,437 discloses a soot blower system in which thermocouples can be placed on either side of a furnace tube or web or embedded in the tube or web. Thermocouples are installed at several locations in the furnace wall zone. Temperatures from all thermocouples are periodically read. All readings taken at a given time are averaged and compared to a selected temperature, such as 710° F. When the temperature average is below that selected temperature soot blowers are activated. There is no teaching to measure or use heat flux. U.S. Pat. No. 4,722,610 to Levert et al. discloses a monitor for measuring slag buildup on furnace walls. A monitoring device is placed on the fire side of the water wall. There is a thermocouple within the device and also within the web. U.S. Pat. No. 4,488,516 to Bueters et al. discloses a soot blower system in which slag on furnace walls is monitored using two thermocouples on the fire side. One thermocouple is placed on a web and the second thermocouple is placed on the surface of a tube. None of these patents correlates slag buildup or presence with slag melting and resultant corrosion activity.
Thermocouples are usually installed on the fire side of a furnace wall in one of two ways. One method is to drill a hole through the web between two furnace wall tubes and insert a thermocouple or other temperature probe through the hole. However, drilling holes affects the strength and integrity of the furnace wall. Consequently, furnace owners are reluctant to do that. The art has also placed temperature probes on the inside of the furnace walls without drilling holes through the furnace wall. These probes and the wires running from them are often welded to the wall surface. The thermocouples or other temperature probes must be shielded with an expensive, high temperature, corrosion resistant material such as Hastelloy or Inconel alloys. Even when such shields are provided the temperature probes have a relatively short useful life. When failure does occur the thermocouple usually cannot practically be replaced until the furnace is shut down for scheduled maintenance.
Within the past fifteen years corrosion engineers have developed probes and methods that can monitor corrosion rates in real time as corrosion is occurring in a variety of equipment. These probes and methods are based upon recognition that corrosion is an electrochemical process during which electrochemical activity is generated. Electrochemical noise is a generic term used to describe low amplitude, low frequency random fluctuations of current and potential observed in electrochemical systems. Thus, by placing electrodes in the corrosive environment, one can measure the electrochemical noise that is present. Hladky in U.S. Pat. No. 4,575,678 discloses that measurements of electrochemical noise in corrosive environments can be used to calculate a rate at which corrosion is occurring. He further discloses an apparatus for measuring corrosion that is occurring in a variety of liquid containing apparatus such as pipes, storage tanks, heat exchangers, pumps and valves. Eden et al. disclose a corrosion monitoring apparatus in U.S. Pat. No. 5,139,627 that also relies upon measurements of electrochemical noise. This apparatus has been commercialized by InterCorr International of Houston, Tex., and is being sold under the name SmartCET system. These devices have been used to measure corrosion in storage tanks and pipes. In those environments there is typically one type of corrosion occurring and temperatures seldom exceed a few hundred degrees. For these systems to be used in a furnace it would be necessary to insert the probes through holes drilled in the furnace wall or shield the probes in the same way thermocouples have been shielded. As previously explained, both alternatives have significant shortcomings.
Consequently, there is a need for a method of determining when and where corrosion is occurring on a furnace wall while the furnace is operating. Such a method should permit the operator of the furnace to adjust the operation of the furnace to reduce or eliminate the corrosion which has been identified.